Numerical experiments for the conductive properties of saturated rock

The reservoir evaluation as a key technology in oil exploration and production is based on the electrical transport property (ETP) of saturated rock that is described in a mathematical form with Arhcie’s equa-tion. But there have been increasing cases observed in many researches indicating that the ETP is non-Archie especially for the complex reservoir with low porosity and permeability. In this paper,the numerical experiments based on the Lattice Boltzmann method (LBM) have been employed to study the effect of porous structure and fluids on the ETP for revealing the nature of non-Archie phenomenon in micro-scale. The results of numerical experiments have proved that the saturation exponent n is a function of water saturation and porosity instead of being a constant in Archie’s equation. And then,a new formula has been developed for the EPT through combining the result of numerical simulation with that of laboratory measurements. The calculations from the new formula show very good agreement with laboratory measurements to demonstrate the efficiency of the new formula over the conventional methods in non-Archie rock.     

Rock physics modeling of heterogeneous carbonatereservoirs： porosity estimation and hydrocarbon detection

In heterogeneous natural gas reservoirs, gas is generally present as small patch-like pockets embedded in the water-saturated host matrix. This type of heterogeneity, alsocalled ＂patchy saturation＂, causes significant seismic velocity dispersion and attenuation. Toestablish the relation between seismic response and type of fluids, we designed a rock physicsmodel for carbonates. First, we performed CT scanning and analysis of the fluid distributionin the partially saturated rocks. Then, we predicted the quantitative relation between the waveresponse at different frequency ranges and the basic lithological properties and pore fluids.A rock physics template was constructed based on thin section analysis of pore structuresand seismic inversion. This approach was applied to the limestone gas reservoirs of the rightbank block of the Amu Darya River. Based on poststack wave impedance and prestack elasticparameter inversions, the seismic data were used to estimate rock porosity and gas saturation.The model results were in ~ood a~reement with the production regime of the wells.     

Evaluation of volcanic reservoirs with the “QAPM mineral model” using a genetic algorithm

Gas-bearing volcanic reservoirs have been found in the deep Songliao Basin, China. Choosing proper interpretation parameters for log evaluation is difficult due to complicated mineral compositions and variable mineral contents. Based on the QAPF classification scheme given by IUGS, we propose a method to determine the mineral contents of volcanic rocks using log data and a genetic algorithm. According to the QAPF scheme, minerals in volcanic rocks are divided into five groups： Q（quartz）, A （Alkaline feldspar）, P （plagioclase）, M （mafic） and F （feldspathoid）. We propose a model called QAPM including porosity for the volumetric analysis of reservoirs. The log response equations for density, apparent neutron porosity, transit time, gamma ray and volume photoelectrical cross section index were first established with the mineral parameters obtained from the Schlumberger handbook of log mineral parameters. Then the volumes of the four minerals in the matrix were calculated using the genetic algorithm （GA）. The calculated porosity, based on the interpretation parameters, can be compared with core porosity, and the rock names given in the paper based on QAPF classification according to the four mineral contents are compatible with those from the chemical analysis of the core samples.     

Organic-rich formation and hydrocarbon enrichment of lacustrine shale strata: A case study of Chang 7 Member

Lacustrine shale is an important target for the exploration of unconventional oil and gas in China beyond marine shale gas.However,the formation environment of lacustrine shale differs from that of marine shale,resulting in a different reservoir composition,organic matter,oil and gas content,and hydrocarbon mobility.In this study,the Chang 7 shale of the Yanchang Formation in the Ordos Basin was used to analyze the effect of volcanic activity on the paleoproductivity and preservation conditions during the formation of lacustrine shale.The results show that algae and bacteria were developed before the eruption.After the eruption,the number of bacteria declined,but the increased prosperity of algae reflects that the volcanic activity enhanced ancient productivity.The sulfate generated by volcanic activity promotes bacterial sulfate reduction,and the produced H₂S leads to a strong reducing environment in the waterbody,which is conducive to the preservation of organic matter.Organic geochemical analysis shows that the black shale in the shale strata has a high total organic carbon(TOC)content and strong hydrocarbon generation potential,whereas the tuff has a low TOC content and can scarcely generate hydrocarbons,indicating that the tuff deposited by volcanic activity cannot be considered as effective source rock.In terms of storage space,shale is mainly laminar and dispersed,and it includes organic and inorganic pores.The development of organic pores is affected by thermal maturity,whereas inorganic pores mainly occur between detrital particles and crystals.Tuff is mainly supported by heterogeneous matrix and associated with alteration.Its pores include inter-and intragranular mineral pores.The development of tight sandstone pores is affected by compaction,cementation,and dissolution,which mainly consist of intra-and intergranular pores.The Chang 7 lacustrine shale generally contains oil,but different lithologies have different oil drainage efficiencies.Sandstone and shale exhibit the best and worst oil drainage efficiency,respectively.It is mainly affected by the pore size distribution,fluid properties,and rock wettability.Therefore,the development of shale oil should mainly focus on lacustrine shale formations with interbeds.The mutual dissolution of organic matter and hydrocarbons in the shale section leads to the poor mobility and difficult development of hydrocarbons.     

：~orous-grain-upper-boundary model and its application to Tarim Basin carbonates

Most of the carbonates in the Tarim Basin in northwest China are low-porosity and low-permeability rocks. Owing to the complexity of porosity in carbonates, conventional rock- physics models do not describe the relation between velocity and porosity for the Tarim Basin carbonates well. We propose the porous-grain-upper-boundary （PGU） model for estimating the relation between velocity and porosity for low-porosity carbonates. In this model, the carbonate sediments are treated as packed media of porous elastic grains, and the carbonate pores are divided into isolated and connected pores The PGU model is modified from the porous-grain-stiff-sand （PGST） model by replacing the critical porosity with the more practical isolated porosity. In the implementation, the effective elastic constants of the porous grains are calculated by using the differential effective medium （DEM） model. Then, the elastic constants of connected porous grains in dry rocks are calculated by using the modified upper Hashin-Shtrikman bound. The application to the Tarim carbonates shows that relative to other conventional effective medium models the PGU model matches the well log data well.     

Seismic response to natural gas anomalies in crystalline rocks

According to the geological and seismic reflection data of the Chinese Continental Scientific Drilling (CCSD) main-hole (MH), and the anomalies of CH4, CO2, and He are correlated to the three-component seismic reflectors, especially in horizontal component profiles. However, the seismic response is dif-ficult to be explained as the porosity of crystalline rocks is only about 1% in well section where the gas anomalies occur. Seismic velocity measurement of the MH cores indicated that compared with wa-ter-saturated rock samples, seismic velocity (especially the S-wave) could be distinctly decreased by gas contained in tiny cracks despite of the low porosity, and then notable seismic response could be induced in gas-filled crystalline rocks. It could be predicated that if the porosity of certain rocks in the middle crust rose due to water-rock interaction and had natural gas filled, then there would be more probability for natural gas in top of the mid-crust to fill in the crystalline rocks with increased porosity. In such case, based on the decrease of Swave velocity in crystalline rocks, seismic method could be applied in the future to explore natural gas reservoirs in the middle crust.     

The formation mechanism of high-quality dolomite reservoir in the deep of Puguang Gas Field

The discovery of Puguang Gas Field provides the exploration of China deep marine carbonate rock with important references.In Puguang Gas Field,the dolomite reservoirs discovered in the deep are the best in the present of China,which present big thickness and wide-range distribution,and develop abundant secondary porosity.The researches show that Puguang Gas Field bears the characteristics of early gas-filling time,deep burial,high matured organic matter and long-term interaction of hydrocarbon(oil and gas)-water-rock(carbonate reservoir).The developments of secondary pores in this area are affected by multiple diagenesis and their formation mechanisms are complicated.Through the research on depositional environment,sedimentary facies and reservoir porosity characters of Changxing and Feixianguan Formations,it is thought that high-quality dolomite reservoirs of Puguang Gas Field form on the favorable sedimentary facies belts,which are the integrate result affected by several factors including superficial corrosion,burial corrosion,overpressure and tectonic movement,among which burial corrosion of TSR to reservoir and overpressure formed by thermal evolution of organic matter have great effect on the formation of secondary porosity of Changxing and Feixianguan Formations.     

天然气水合物似海底反射层(BSR)AVA特征:双相介质模型

The bottom simulating reflector （BSR） in gas hydrate-bearing sediments is a physical interface which is composed of solid, gas, and liquid and is influenced by temperature and pressure. Deep sea floor sediment is a porous, unconsolidated, fluid saturated media. Therefore, the reflection and transmission coefficients computed by the Zoeppritz equation based on elastic media do not match reality. In this paper, a two-phase media model is applied to study the reflection and transmission at the bottom simulating reflector in order to find an accurate wave propagation energy distribution and the relationship between reflection and transmission and fluid saturation on the BSR. The numerical experiments show that the type I compressional （fast） and shear waves are not sensitive to frequency variation and the velocities change slowly over the whole frequency range. However, type II compressional （slow） waves are more sensitive to frequency variation and the velocities change over a large range. We find that reflection and transmission coefficients change with the amount of hydrate and free gas. Frequency, pore fluid saturation, and incident angle have different impacts on the reflection and transmission coefficients. We can use these characteristics to estimate gas hydrate saturation or detect lithological variations in the gas hydrate-bearing sediments.     

Finite-difference modeling with variable grid-size and adaptive time-step in porous media

Forward modeling of elastic wave propagation in porous media has great importance for understanding and interpreting the influences of rock properties on characteristics of seismic wavefield. However,the finite-difference forward-modeling method is usually implemented with global spatial grid-size and time-step; it consumes large amounts of computational cost when small-scaled oil/gas-bearing structures or large velocity-contrast exist underground. To overcome this handicap,combined with variable grid-size and time-step,this paper developed a staggered-grid finite-difference scheme for elastic wave modeling in porous media. Variable finite-difference coefficients and wavefield interpolation were used to realize the transition of wave propagation between regions of different grid-size. The accuracy and efficiency of the algorithm were shown by numerical examples. The proposed method is advanced with low computational cost in elastic wave simulation for heterogeneous oil/gas reservoirs.     

Effect of thermal stimulation on gas production from hydrate deposits in Shenhu area of the South China Sea

The Shenhu area on the northern continental slope of the South China Sea (SCS) is one of the promising fields for gas hydrate exploitation. The hydrate-bearing layer at drilling site SH2 is overlain and underlain by permeable zones of mobile water. In this study a vertical well was configured with a perforated Interval I for producing gas and a coiled Interval II for heating sediment. The hydrate is dissociated by a small depressurization at Interval I and a thermal stimulation at Interval II. The numerical simulations indicate that the thermal stimulation has a significant effect on gas release from the hydrates in the production duration and improves the gas production in the late period. The gas released by thermal stimulation cannot be produced as quickly as the production gets operated because of the hard pathway for fluids to flow in the sediments. The gas production is enhanced due to the heating for 7242 m 3 in the whole production. Increasing heating temperature at Interval II can improve gas production and restrain water output, and advance the arrival time of the gas flow from the zone at Interval II. The absolute criterion and relative criterion suggest that the thermal stimulation in the production schemes is pronounced for releasing gas from the hydrate deposit, but the production efficiency of gas is limited by the sediment of low permeability. The study provides an insight into the production potential of the hydrate accumulations by thermal stimulation with depressurization in two wells, and a basis for analyzing economic feasibility of gas production from the area.     

Determination of the elastic wave velocities in porous rocks with the change of overburden pressure and its universal significance

Typical rock samples with different lithologic characteristics were collected from exploring wells drilled in sandstone-conglomerate sedimental reservoirs with positive rhythm. In different pore fluid states (fully saturated with gas, water and oil), the velocities of compressional and shear waves (Vp, Vs) were measured under different overburden pressure in laboratory. The effects of pore fluid and different fluid types on the velocities were analyzed. The velocities (Vp, Vs) of the samples fully saturated with water were calculated by use of Gassmann's formula that is suitable for low frequency. The calculated values were compared with the experimental values obtained at high frequency. The result shows that Gassmann's theory can be used to calculate elastic wave velocities in porous rocks saturated with fluid. By this result, the change of elastic velocities with the change of fluid can be predicted. The error is allowable in petroleum engineering. This conclusion is useful for sonic logging interpretation and seismic datum processing.     

高温高压下岩石弹性波速特征及其在深部地质研究中的意义

本文收集、整理、研究了７０年代以来的高温高压下岩石的Ｖｐ，Ｖｓ，Ｖｐ／Ｖｓ，Ｖｐ各向异性等实验资料后得出：（１）低压下岩石Ｖｐ的急剧变化是由岩石中裂纹与孔隙等引起的；（２）温度升高可增大由岩石组构引起的Ｖｐ各向异性的变化率；（３）同种岩石在不同的大地构造环境中具有不同的波速值与变化规律；（４）Ｖｐ在高地温区随温度先降低而后升高，其原因是由矿物相变造成的，与地壳中低速带的性质相当。文章最后讨论了实测波速转换为岩石类型时实验研究岩石波速的意义。     

Modulus defect,velocity dispersion and attenuation in partially-saturated reservoirs of Jurassic sandstone,Indus Basin,Pakistan

Partially saturated reservoirs are one of the major sources of seismic wave attenuation, modulus defect and velocity dispersion in real seismic data. The main attenuation and dispersion phenomenon is wave induced fluid flow due to the heterogeneity in pore fluids or porous rock. The identification of pore fluid type, saturation and distribution pattern within the pore space is of great significance as several seismic and petrophysical properties of porous rocks are largely affected by fluid type, saturation and fluid distribution pattern. Based on Gassmann-Wood and Gassmann- Hill rock physics models modulus defect, velocity dispersion and attenuation in Jurassic siliclastic partially-saturated rocks are studied. For this purpose two saturation patterns - uniform and patchy - are considered within the pore spaces in two frequency regimes i.e., lower frequency and higher frequency. The results reveal that at low enough frequency where saturation of liquid and gas is uniform, the seismic velocity and bulk modulus are lower than at higher frequency where saturation of fluid mixture is in the form of patches. The velocity dispersion and attenuation is also modeled at different levels of gas saturation. It is found that the maximum attenuation and velocity dispersion is at low gas saturation. Therefore, the dispersion and attenuation can provide a potential way to predict gas saturation and can be used as a property to differentiate low from high gas saturation.     

Simultaneous measurements of compressional wave and shear wave velocities, Poisson's ratio, and Vp/Vs under deep crustal pressure and temperature conditions: Example of silicified pelitic schist from Ryoke Belt, Southwest Japan

The ultrasonic technique for measuring travel times of compressional and shear waves using dual-mode transducers was adapted to a piston cylinder apparatus, allowing simultaneous measurements of travel times of compressional and shear waves of island arc samples under the high pressure and temperature conditions of island arcs. This method enables us to determine elastic properties and their pressure and temperature derivatives simultaneously. Furthermore, Vp/Vs can be directly determined from travel times of compressional and shear waves independently of length change due to compression or thermal expansion of rock samples under deep crustal conditions, providing more accurate Vp/Vs values than those determined from individual measurements of travel times of both elastic wave types using single-mode transducers. Experimental techniques and results are demonstrated using data on silicified pelitic schist from the Ryoke Belt to 0.6 GPa. The simultaneous measurement gives Vp  = 5.60 km/s, ∂ Vp /∂ P  = 0.090 (km/s)/GPa, Vs  = 3.37 km/s, ∂ Vs /∂ P  = 0.05 (km/s)/GPa, σ  = 0.216, and Vp / Vs  = 1.66 at ambient conditions. The temperature derivatives were constrained from fitting using linear functions of temperature, yielding ∂ Vp /∂ T  = −0.518 × 10⁻³ (km/s)/K and ∂ Vs /∂ T  = −0.182 × 10⁻³ (km/s)/K. Performing simultaneous measurements of travel times of compressional and shear waves using dual-mode transducers, it is possible to accurately determine Vp / Vs and Poisson's ratio of crustal minerals and rocks at deep crustal conditions to study the composition of the crustal interior, e.g. rock types and fluids below the hypocentral region of earthquakes or around bright spots.     

基于常规测井数据的纵波本征衰减估计

地震波本征衰减反映了地层及其所含流体的一些特性,对油气勘探开发有重要意义.已有的理论研究与实验发现,地震频带内的衰减主要与中观尺度(波长与颗粒尺度之间)的斑状部分饱和、完全饱和岩石弹性非均匀性情况下波诱导的局部流体流有关.这种衰减与岩石骨架、孔隙度及充填流体的性质密切相关.本文着重讨论均匀流体分布、斑状或非均匀流体分布两种情况下部分饱和岩石的纵波模量差异.以经典岩石物理理论和衰减机制认识为基础,通过分析低频松弛状态、高频非松弛状态岩石的弹性模量,讨论储层参数(如孔隙度、泥质含量以及含水饱和度等)与纵波衰减之间的确定性关系.上述方法与模型在陆相砂泥岩地层与海相碳酸盐岩地层中的适用性通过常规测井资料得到了初步验证.     

含湿孔隙岩石有效热导率的数值分析

本文采用有限元方法研究含湿孔隙岩石的有效热导率,即随机划分网格并指定材料性质,建立三维含湿孔隙岩石的有限元模型,模型的上下表面施加不同的温度,侧面绝热,计算出总热流,然后结合上下表面的温度梯度计算出岩石的有效热导率.考虑到单个随机模型不一定具有代表性,对给定的孔隙率和饱和度均生成了200种矿物、水、空气随机分布的岩石模型,进行Monte Carlo实验和统计分析,统计分析结果与前人实验结果吻合良好.数值分析结果表明,孔隙岩石的有效热导率与岩石的孔隙率、饱和度、固体矿物组分及孔隙的分布情况有关,数值计算的误差随着网格数目的增加而减小.此有限元方法可以用来估算岩石的有效热导率,在已知组分性质的多矿物岩石物性计算方面有广阔应用前景.     

含裂隙介质受压破裂前地震波特性的综合实验研究与理论分析

本文用单轴压缩下砂岩试件的P、s波超声实验,对含裂隙高空度砂岩破裂前的Vp、Vs、Vp/Vs、Ap、As、As/Ap,初动段频谱,介质传递函数作了一系列实验研究。结果表明:l.Vp、Vs及Vp/Vs的变化同花岗岩,2.As/Ap要发生变化,3.有低频增加现象,4.介质传递函数发生变化,5.有第二P波产生。最后文章用两相介质中地宸波传播理论对实验结果作了定量分析。     

Seismic imaging of the middle and upper crust by double-difference tomography: the Haicheng earthquake (Ms 7.3) in Liaoning Province

The Haicheng earthquake (Ms 7.3) occurred in Liaoning Province (39°N–43°N, 120°E–126°E ), China on February 4, 1975. The mortality rate was only 0.02% owing to the first timely and accurate prediction, although the area affected by the earthquake was 9200 km² and covered cities with a population density of 1000 p/km². In this study, the doubledifference (DD) tomography method was used to obtain high-resolution three-dimensional (3D) P- and S-wave velocity (Vp and Vs) structures and Vp/Vs as well as the earthquake locations. Tomography results suggest that velocity structure at shallow depth coincides well with topography and sediment thickness. The earthquake locations form a northwest-striking zone associated with the Jinzhou(JZ) Fault and a northeast-striking zone associated with the Haichenghe-Dayanghe (HD) Fault, and suggest that the JZ Fault consists of three faults and the Ms 7.3 Haicheng earthquake originated at the intersection of the JZ and the Faults. Lowvelocity zones (LVZs) with low Vp/Vs are observed at 15–20 km depth beneath the Haicheng (HC) region. We interpret the LVZs in the middle crust as regions of fluids, suggesting rock dehydration at high temperatures. The LVZs and low Vp/Vs in the upper crust are attributed to groundwater-filled cracks and pores. We believe that large crustal earthquakes in this area are caused by the combination of faulting and fluid movement in the middle crust.     

Multiscale rock-physics templates for gas detection in carbonate reservoirs

The heterogeneous distribution of fluids in patchy-saturated rocks generates significant velocity dispersion and attenuation of seismic waves. The mesoscopic Biot–Rayleigh theory is used to investigate the relations between wave responses and reservoir fluids. Multiscale theoretical modeling of rock physics is performed for gas/water saturated carbonate reservoirs. Comparisons with laboratory measurements, log and seismic data validate the rock physics template. Using post-stack and pre-stack seismic inversion, direct estimates of rock porosity and gas saturation of reservoirs are obtained, which are in good agreement with oil production tests of the wells.     

汶川地震破裂带断层岩纵波速度与孔隙度关系的实验研究

分布于地震破裂带上的断层岩具有高孔隙度的特征.该特点造成了其弹性波速度与结晶岩石和沉积岩存在明显的差异.确定断层岩的弹性波速度与孔隙度和矿物组成的关系对于利用地震资料探测深部断层和测井资料的解释至关重要.在10~600 MPa条件下,本文对地震断层岩的纵波波速（V_p）和总孔隙度（φ_t）进行了测量,并深入分析了V_p与孔隙度的关系.结果表明在10~600 MPa的压力范围内,V_p（p）随着压力的增高呈现对数增加,其增长率随着压力的上升而逐渐减小,遵从∂V_p（p）/∂p=a_v/p的变化规律.断层岩中的孔隙度随着压力的增高呈对数减小.与传统的认识不同,实验发现在压力高达600 MPa以上,大多数断层岩中仍然可以残留可观的孔隙量.分析显示V_p与总孔隙度及总粘土含量呈负线性相关.该发现有助于认识深部流体的活动通道特征,有助于理解断层带中存在大量粘土矿物、断层带内的物质可被大量带出、围陷波的形成等地质和地球物理现象.